Producing of free-machining stainless steel having a low sulphur content



Patented Oct. 22 1940 raonucnve F FREE-MACHINING scram- LESS STEEL CONTENT HAVING A LOW SULPHUR Vincent 'T. Malco1m, Indian Orchard, Mass, assignor to.The Chapman Valve Manufacturing 00., Indian Orchard, Mass., a corporation of Massachusetts No Drawing.- Application October- 18, 1939, .Serial No. 300,01 6

2 Claims.

This invention relates to improvements in producing a free-machining stainless steel of the ferritic type having a low sulphur content and'is directed more particularly to the pro- 5 vision of a free-machining stainless steel having all the desirable characteristics inherent in steels of the same type which have a relatively high sulphur content but none of their disadvantages. A

It has for. some time been the practice to make stainless steels of the ferritic type for use in the manufacture of such machined parts as valve stems, other valve parts, etc., by means of r "such machines as so-called automatic screw machines which operate at high speeds, but success hasonly been had, as far as efliciency, speed, and economy are concerned, when these steels have had a sulphur content of approximately .25 to 50%; That is to say, stainless steels of the ferritic type having what is considered a relatively high sulphur content, that is between .25 'to 50%, whenoperated upon in automatic screw machines give up chips which are cut or turned short and which are brittle so as to break away rapidly from the tool, with the result that the tool is kept clean and rapid, free cutting or turning is possible.

This, of course, is all as desired since the attainment of speedy efliciency is obviously the reason for the development and use of automatic machines.

However, the oil, chemical, and other indus machining, the sulphur content at the same time has been more or less of a contaminating element wherefore such steels are not satisfactory for use in these industries for the reasons stated.

Other stainless steels of the ferfitic type containing relatively less sulphur have, of course, been produced but they are not readily adapted for machining inautomatics. That is to say, stainless steels containingless than about .05% sulphur have been made but they have never been used with success in automatic machines which operate at high speeds-since the cutting speed of such a low-sulphur-content. stainless steel is about 30 feet per minute as, contrasted sulphur content are capable of rapid and efficient with a cutting speed-oi around feet per minute of a stainless steel containing the larger amount of sulphur above mentioned.

Further, the outside threading and finishing of a stainless steel having a low sulphur content on an automatic high-speed machine is rough 6 and of poor quality and tapping thereon is practically impossible. That is, such steels have had to be removed from the automatic machines andtapped and threaded by more or less of a hand method. 10 A further disadvantageof stainless steels of the ferritic type which have purposely been made with a relatively low sulphur content in order to satisfy the industries mentioned is that the life of the tools is shortened about 50% and regrinding of'the tools is necessary after working on five lengths of twelve feet as contrasted with every thirty such bars as previously. This, of

- course, is a serious objection and defeats the very purpose of high-speed automatic machining.

It is, accordingly, the principal object of this invention'to provide a stainless steel of the ferritic type which does not have the corrosive" or objectionably brittle qualities of such steels containing the relatively high amount of sulphur but which has their fine machining qualities. In other words, by the method of. this invention, I take a stainless steel of the ferritic type having a low sulphur content and give to it the machining properties of a similar steel having a higher sulphur content while eliminating the contaminating influence thereof.

The formula of a common stainless steel of the ferritic type having a relatively high sulphur content and which has met with success as far as its machinability is concerned, is as follows;

Percent Carbon maxim Manganese maximum Silicon maximum Phosphorus maximum .04

Nickel maximum .50 Chromium maxim 11.5-13.0 Sulphur approximately .25 to .50

above except that the sulphur content of the steel I work has a maximum sulphur content 01 .05%. o a

-essed according to this novel manner of treating the steel work with heat and, because of its nature, no subsequent temering or drawing is required. As a first step, I preferably heat the steel to a temperature of from 1500 to 1600 F., and preferably to about 1550 F., and soak it a suflicient length of time until the heat penetration is complete.

The steel is then allowed to cool slowly with the furnace. As the next step I reheat the steel to a temperature within range of from 1550 to 1625" F.

The preferred reheating temperature is about 1580 F. and the steel is allowed to soak in this heat long enough for complete penetration. The

final step consists in allowing the work to cool rapidly in air.-

If desired, the first step may be eliminated. That is the initial heating to between 1500 and 1600 F. and slow cooling may be omitted and the treatment confined to the heating to saturation at about 1580" F. and cooling rapidly in air.

Such a treatment, I have found, results in a stainless ferritic steel of low sulphur content having a cutting speed in an automatic screw machine increased from 30 feet to 140 feet per minute. Further, steel processed according to this invention, notwithstanding the fact that it is of low sulphur content, has a tool-life times greater than the tool-life of steels heretofore known.

It has also been found that with steels procinvention, tapping. threading, and finishing may be eificiently done on automatic machines operating at high speed, thereby eliminating separate operations. Still further, the times of regrinds of tools operating on steels of this invention is increased about 400% and one automatic machine working thereon will do as much machining and'more efficiently, as did three machines on steels of the previous practice.

Steels'subjected to my special treatment developed all th desired physical properties such as tensile strength, impact value, etc., in aldegree equal to, if not superior to steels subjected to orthodox treatment, ,It is also a fact that my method of treating low-sulphur-content steels such as I have described results in the same having a hardness of least 1700 and quench in oil, then reheat to a suitable temperature (known as drawing or the like) for the desired physical properties. This latter temperature may range between 400-l200 F. and while, by this treatment, Brinell hardness numbers of 220 to 360 may be obtained, the steel is non-machinable in automatic screw machines.

In other words, the steels of this invention have the advantage of being considerably harder than 'most of those heretofore known but at the same time they are readily machinable and non-corro'sive.

While I have described the invention in great detail and with respect to the present preferred form thereof, it is not desired to be limited thereto since changes and modifications may be made therein without departing from thespirit and scope of the invention. The invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments are therefore to be considered in all respect as illustrative and not restrictive, the scope of the invention being in 'cated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. v

What it is desired to claim and secure by Letters Patent of the United States is: I

l. The steps in the process of producing freemachining ferritic stainless steel having less than .0 5% sulphur consisting of, heating the steel to a temperature within a range of from 1500 F. to 1600" F., allowing it to cool slowly, reheating the Steel to a temperature within a range of from 1550 F. to 1625 F., and then cooling the steel rapidly in air.

2. The steps in the process of producing freemachining ferritic stainless steel having approximately .05% sulphur consisting of, heating the steel in a furnace to about 1550 F., allowing it to cool slowly within the furnace, reheating the steel to about 1580 F., and then cooling the steel rapidly in air. Y I

. VINCENT T. MALCOLM. 

